I'm not sure this answer is universal, but based on the one I'm unfortunately related to and have to talk to from time to time, libertarians would say that regulation is preventing other truck manufacturers from entering and competing in the market. If the market were "truly" free, then a competitor could come along and do better by selling their vehicles at a lower interest rate, taking all the business away from the interest-rate-abusing company. If you try to explain that this problem is not due to regulation but due to a lack of it, or how trusts happen and how monopolies work, they will fall back on the same set of naive circular reasoning. It doesn't have to make sense or reflect reality, it just has to vaguely sound plausible and fit with what they want to be true.